A white-light interferometric measuring device using broadband light (white light) as a light source is widely known as one of conventional interferometric measuring devices for measuring the three-dimensional shape of a measurement workpiece accurately by using an intensity information about an interference pattern generated by interference of light. As an example thereof, FIG. 8 shows the outline of the structure of a white-light interferometric measuring device described in Japanese Patent Application Laid-Open No. 2011-85413 (hereinafter called patent literature 1). The white-light interferometric measuring device of FIG. 8 includes a white light source 6 that emits a white light beam 8, a beam splitter 16 that reflects the white light beam 8, and an interference objective lens 20. The interference objective lens 20 collects the white light beam 8 having reflected off the beam splitter 16 in the direction of an optical axis O and irradiates a measurement workpiece 2 with the white light beam 8. The interference objective lens 20 also generates interference between a measurement light beam obtained by reflection of the white light beam 8 off the measurement workpiece 2 and a reference light beam obtained by branching of the white light beam 8 to be converged on the measurement workpiece 2. Reference numerals 10, 22, 24 and 26 indicate a collimator lens, an interference light beam, an imaging lens, and a light receiving element.
A description will be given of the fact that high resolution can be achieved in a height direction (Z direction) by using interference of white light, with reference to FIGS. 9A and 9B. FIG. 9A shows intensity distributions of interference patterns in the Z direction obtained at respective wavelengths of white light. FIG. 9B shows an intensity distribution of an interference pattern in the Z direction obtained as a result of combination of the interference patterns of all the wavelengths. As shown in FIG. 9A, the maximum values of the interference patterns of the respective wavelengths overlap at a focal position of the interference objective lens 20 in the Z direction (Z=0), and the phases of the respective wavelengths shift more with a greater distance from the focal position. Therefore, as shown in FIG. 9B, the intensity of the combined interference pattern becomes maximum at the focal position, and becomes lower gradually while oscillating with a greater distance from the focal position. Thus, the white-light interferometric measuring device is capable of measuring the three-dimensional shape of the measurement workpiece 2 accurately within the field of view of the interference objective lens 20 by detecting a position in the Z direction where the intensity becomes maximum at each position within the field of view of the interference objective lens 20.